Optical technology utilizes optical signals in order to carry information. Typically, the signals are carried on different channels. Each channel typically includes a small range of wavelengths centered on a characteristic wavelength. Often an optical signal is a composite signal including multiple channels. Because each channel has its own characteristic wavelengths, multiple channels can be carried at the same time by the same component, such as a fiber. As the use of optical technology increases, the number of signals transmitted on a single fiber also increases. Currently, optical signals can include twenty, forty, or more channels.
Optical technology also utilizes optical filters for a variety of applications. Filters transmit light in a pass band. The pass band of the filter is a range of wavelengths centered on a central wavelength. One application for filters is in demultiplexing an optical signal carrying multiple channels. In order to create the optical signal, channels at a plurality of characteristic wavelengths are multiplexed together. A single cable may then be used for transmission. After the multiplexed, multi-channel signal reaches the destination, the channels are demultiplexed to access the information carried by a single channel. Conventional optical filters can be used to combine individual signals as well as to separate a composite signal into individual channels.
In order to separate a composite signal into its component channels, a plurality of conventional optical filters, one for each channel, are cascaded in series. Each optical filter includes an input, usually a fiber, which is held in place by a holder, such as a glass capillary. The optical filter is transmitted to a lens, which collimates the signal, and a filter. The lens is typically a GRIN (Graduated Index of refraction) lens. The filter passes light centered on a single wavelength. The filtered light is then typically provided to a second lens and output over a second fiber held in place by a second holder. Thus, each optical filter outputs a single channel. As each component wavelength is separated from the composite signal, the remaining portion of the composite signal is passed on to the next optical filter in the series. Thus, the signal is separated into its components. Note that other applications may also use optical filters. In general, optical filters are useful in applications where it is desired to remove a portion of the signal outside of the filter's pass band. Thus, filters may be used in noise removal or other applications.
Although conventional systems for filtering signals are capable of transmitting light in the pass band, a conventional filter is typically fixed to the end of the GRIN lens. The angle of incidence of the signal is the angle between the direction of propagation of the signal and the normal to the surface. Thus, the angle of incidence of the optical signal for the filter is fixed. The signal from the GRIN lens also typically passes parallel to the axis of the GRIN lens. Thus, the angle of incidence of the optical signal at the surface of the filter is typically at approximately zero degrees. The central wavelength for the pass band is, therefore, also fixed. When manufacturing the optical filter, therefore, the filter used must be carefully selected to have the desired central wavelength. As a result, the filter itself will be subject to tighter specifications in order to ensure that the appropriate portion of the signal is transmitted. The tighter specifications make the filter more expensive and difficult to make. In addition, one filter is typically used for each channel. As a result, the cost of providing optical filters for a composite signal is high. This is true because the cost of producing filters with different specifications is higher than the cost of producing filters having the same specifications. In addition, the filters will occupy a relatively large space.
Accordingly, what is needed is a system and method for providing an optical filter which is low cost, relatively easy to manufacture, and relatively accurate. The present invention addresses such a need.